1. Technical Field
This disclosure generally relates to oil and gas well drilling and the subsequent investigation of subterranean formations surrounding the well. More particularly, this disclosure relates to apparatus and methods for obtaining sidewall cores from a subterranean formation.
2. Description of the Related Art
Wells are generally drilled into the ground or ocean bed to recover natural deposits of oil and gas, as well as other desirable materials that are trapped in geological formations in the Earth's crust. A well is typically drilled using a drill bit attached to the lower end of a “drill string.” Drilling fluid, or “mud,” is typically pumped down through the drill string to the drill bit. The drilling fluid lubricates and cools the drill bit, and it carries drill cuttings back to the surface in the annulus between the drill string and the wellbore wall.
Once a formation of interest is reached, drillers often investigate the formation and its contents through the use of downhole formation evaluation tools. Some types of formation evaluation tools form part of the drill string and are used during the drilling process. These are called, for example, “logging-while-drilling” (“LWD”) tools or “measurement-while-drilling” (“MWD”) tools. MWD typically refers to measuring the drill bit trajectory as well as wellbore temperature and pressure, while LWD refers to measuring formation parameters or properties, such as resistivity, porosity, permeability, and sonic velocity, among others. Real-time data, such as the formation pressure, allows the drilling company to make decisions about drilling mud weight and composition, as well as decisions about drilling rate and weight-on-bit, during the drilling process. While LWD and MWD have different meanings to those of ordinary skill in the art, that distinction is not germane to this disclosure, and therefore this disclosure does not distinguish between the two terms. Furthermore, LWD and MWD are not necessarily performed while the drill bit is actually cutting through the formation. For example, LWD and MWD may occur during interruptions in the drilling process, such as when the drill bit is briefly stopped to take measurements, after which drilling resumes. Measurements taken during intermittent breaks in drilling are still considered to be made “while-drilling” because they do not require the drill string to be removed from the wellbore, or “tripped.”
Other formation evaluation tools are used sometime after the well has been drilled. Typically, these tools are lowered into a well using a wireline for electronic communication and power transmission, and therefore are commonly referred to as “wireline” tools. In general, a wireline tool is lowered into a well so that it can measure formation properties at desired depths.
One type of wireline tool is called a “formation testing tool.” The term “formation testing tool” is used to describe a formation evaluation tool that is able to draw fluid from the formation into the downhole tool. In practice, a formation testing tool may involve many formation evaluation functions, such as the ability to take measurements (i.e., fluid pressure and temperature), process data and/or take and store samples of the formation fluid. Thus, in this disclosure, the term formation testing tool encompasses a downhole tool that draws fluid from a formation into the downhole tool for evaluation, whether or not the tool stores samples. Examples of formation testing tools are shown and described in U.S. Pat. Nos. 4,860,581 and 4,936,139, both assigned to the assignee of the present application.
During formation testing operations, downhole fluid is typically drawn into the downhole tool and measured, analyzed, captured and/or released. In cases where fluid (usually formation fluid) is captured, sometimes referred to as “fluid sampling,” fluid is typically drawn into a sample chamber and transported to the surface for further analysis (often at a laboratory). As fluid is drawn into the tool, various measurements of downhole fluids are typically performed to determine formation properties and conditions, such as the fluid pressure in the formation, the permeability of the formation and the bubble point of the formation fluid. The permeability refers to the flow potential of the formation. A high permeability corresponds to a low resistance to fluid flow. The bubble point refers to the fluid pressure at which dissolved gasses will bubble out of the formation fluid. These and other properties may be important in making downhole decisions.
Another downhole tool typically deployed into a wellbore via a wireline is called a “coring tool.” Unlike the formation testing tools, which are used primarily to collect sample fluids, a coring tool is used to obtain a sample of the formation rock.
A typical coring tool includes a hollow drill bit, called a “coring bit,” that is advanced into the formation wall so that a sample, called a “core sample,” may be removed from the formation. A core sample may then be transported to the surface, where it may be analyzed to assess, among other things, the reservoir storage capacity (called porosity) and permeability of the material that makes up the formation; the chemical and mineral composition of the fluids and mineral deposits contained in the pores of the formation; and/or the irreducible water content of the formation material. The information obtained from analysis of a core sample may also be used to make downhole decisions.
Downhole coring operations generally fall into two categories: axial and sidewall coring. “Axial coring,” or conventional coring, involves applying an axial force to advance a coring bit into the bottom of the well. Typically, this is done after the drill string has been removed, or “tripped,” from the wellbore, and a rotary coring bit with a hollow interior for receiving the core sample is lowered into the well on the end of the drill string. An example of an axial coring tool is depicted in U.S. Pat. No. 6,006,844, assigned to Baker Hughes.
By contrast, in “sidewall coring,” the coring bit is extended radially from the downhole tool and advanced through the side wall of a drilled borehole. In sidewall coring, the drill string typically cannot be used to rotate the coring bit, nor can it provide the weight required to drive the bit into the formation. Instead, the coring tool itself must generate both the torque that causes the rotary motion of the coring bit and the axial force, called weight-on-bit (“WOB”), necessary to drive the coring bit into the formation. Another challenge of sidewall coring relates to the dimensional limitations of the borehole. The available space is limited by the diameter of the borehole. There must be enough space to house the devices to operate the coring bit and enough space to withdraw and store a core sample. A typical sidewall core sample is about 1.5 inches (about.3.8 cm) in diameter and less than 3 inches long (.about.7.6 cm), although the sizes may vary with the size of the borehole. Examples of sidewall coring tools are shown and described in U.S. Pat. Nos. 4,714,119 and 5,667,025, both assigned to the assignee of the present application.
During sidewall core analysis, it is advantageous to know the orientation of the core as it resided in the formation prior to its removal. “Orientation” as used herein means which end of the core faced or was exposed to the borehole. Additionally or alternatively, the “orientation” of a core indicates how the core was positioned with respect to the axis of the borehole (i.e., which part of the core was at the least depth or top). Currently, sidewall core orientation can be determined by a close examination of the physical features of the core. This method, however, requires an intimate knowledge of the formation geology as well as the operation of the coring tool. The details of the formation geology are often not known or overly expensive to obtain, and therefore this approach is not feasible in many applications. In some circumstances, “Orientation” could refer to drilling orientation, or the radial direction, relative to the center of the borehole, in which the core was taken. Projected on a horizontal plane, this type of orientation is typically measured in degrees from North. Drilling orientation measurements are already possible with the use of downhole orientation tools.